problem shooting auto exposure bracket sets

-2 EV, first shot of 3 bracket set

0EV, second shot of three bracket set

+2EV, third shot of three bracket set demonstrating a slight fish eye effect.

When shooting AE brackets sets with the nx200 and 18-200mm, about 20% of the sets come out with the 3rd and last shot with a slight fish-eye effect. I cant seem to reproduce the effect consistently. May anyone have had a similar experience or have an understanding of what could be the culprit?